Curl from container A to B blocked

Traefik Traefik v1
14 
# curl -v -H "Host: a.example.com" http://traefik:80
* Rebuilt URL to: http://traefik:80/
* Hostname was NOT found in DNS cache
*   Trying 172.27.0.4...
* Connected to traefik (172.27.0.4) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.38.0
> Accept: */*
> Host: a.example.com
> 
< HTTP/1.1 302 Found
< Location: https://a.example.com:443/
< Date: Sat, 27 Jul 2019 11:13:40 GMT
< Content-Length: 5
< Content-Type: text/plain; charset=utf-8
< 
* Connection #0 to host traefik left intact